An experimental study of pyroxene crystallization during rapid cooling in a thermal gradient; applications to komatiites and chondrites

Abstract. To investigate the crystallization of pyroxene in spinifex-textured komatiites and in chondrites we undertook a series of experiments in which compositions in the CMAS system were cooling rapidly in a thermal gradient. Cooling rates were generally between 5 to 10 °C h−1 but some runs were made at 100–200 °C h−1; thermal gradients were between 10 and 20 °C cm−1. These conditions reproduced those at various levels in the crust of komatiitic lava flow. The starting composition was chosen to have pigeonite on the liquidus and a majority of the experiments crystallized zoned pigeonite-diopside crystals like those in komatiite lavas. A~conspicuous aspect of the experimental results was their lack of reproduceability. Some experiments crystallized forsterite whereas others that were run under similar conditions crystallized two pyroxenes and no forsterite; some experiments were totally glassy but others totally crystallized to pyroxene. The degree of supercooling at the onset of pyroxene crystallization was variable, from less than 25 °C to more than 110 °C. We attribute these results to the difficulty of nucleation of pyroxene. In some cases forsterite crystallized metastably and modified the liquid composition to inhibit pyroxene crystallization; in others no nucleation took place until a large degree of supercooling was achieved, then pyroxene crystallized rapidly. Pigeonite crystallized under a wide range of conditions, at cooling rates from 3 to 100 °C h−1. The notion that this mineral only forms at low cooling rates is not correct.